The invention is directed to the preparation of polymer laminated base paper and more in particular to the preparation of polymer laminated photographic base paper.
Polymer laminated base paper for photographic printing paper is conventionally prepared from a base paper that is laminated with at least one polymer resin layer, usually a titanium dioxide filled polyethylene, polypropylene or a polymethylene-methacrylate resin, by extrusion-coating (or co-extrusion-coating) of a polymer melt onto the surface of the base paper.
In JP-A 57-102622 a process is described for the extrusion-coating of a base paper with polyethylene, in which process the base paper is subjected to a corona treatment, followed by heating to a temperature of at least 80xc2x0 C. This process aims at improving the adhesion between base paper and polymer coating.
An important aspect of the quality of polymer resin laminated base paper is the resin surface appearance, which should not have a large number of crater-like defects (or pits) after extrusion-coating of the polymer resin on the base paper.
Further important aspects of photographic base paper are the speed of production and the thickness of the polymer layers. Both aspects are important in view of the economy of the process of producing photographic print paper. The formation of pits has turned out to be strongly dependent on those aspects.
It has been attempted to suppress the number of pits by increasing the thickness of the polymer resin layer. At high extrusion-coating speeds, such as over 300 m/min this is not sufficiently effective, unless very large amounts of polymer are used. Furthermore, it has economical disadvantages as the polymer resin is an important cost factor in the production of photographic print paper.
At extrusion-coating speeds of the polymer resin of 300 m/min or more it has been found to be much more difficult to reduce the number of pits.
In the art of providing polymer laminated photographic base paper there is a need for high speed production, especially during the extrusion-coating of the paper with the polymer resin, whereby the amount of pits remains at a sufficiently low value.
Accordingly it is an object of the present invention to provide a process for the preparation of a polymer laminated base paper, more in particular a polymer laminated photographic base paper, wherein the number of pits is decreased, especially at higher extrusion-coating speeds.
It is a further object of the invention to provide a process for the preparation of a polymer laminated base paper, more in particular a polymer laminated photographic base paper, wherein the number of pits is decreased, at higher extrusion-coating speeds, without the need to increase the polymer weight.
The present invention is based thereon, that the coated paper shows less pits in case the base paper web is heated, preferably by infrared radiation, prior to the extrusion coating with the polymer resin.
Accordingly the present invention is directed to a process for the preparation of polymer laminated base paper, said process comprising laminating a base paper web on at least one side thereof with at least one polymer layer, by extrusion-coating at a coating speed of 300 m/min. or more, wherein the said base paper web is heated prior to laminating.
Surprisingly it has been found that with this process the number of pits can be reduced, even at high extrusion-coating speeds, such as extrusion-coating speeds in excess of 300 m/min. The advantages of the invention become apparent already at extrusion speeds of 300 m/min or over. It is preferred to use a speed of at least 350 m/min, more in particular at least 400 m/min. Presently an upper limit of 700 m/min is considered acceptable.
In the broadest sense the present invention resides therein that the paper web is heated just prior to the (co-)extrusion-coating. This heating can be done by the application of heated air, heated rollers and/or by radiation. More in particular, preference is given to the use of radiation in the Near Infra Red (NIR) region. In particular, it is to be noted that the NIR radiation can be emitted at radiation temperatures of over 2500 K, preferably over 2900 K, most preferred over 3000 K. Generally, the upper limit for the emission temperature is 3500 K.
It has been found that surprisingly the use of NIR radiation provides good results in terms of product properties. Further, it has been found that in terms of heating rate the use of NIR has advantages.
The advantages of the treatment of the web by infrared radiation, with wavelength ranges between 0.8 xcexcm and 1 mm, results in the increase of the web temperature in an extremely short treatment period, usually less than 1 second. With the use of conventional heating technologies this is less easy too achieve. The fastest temperature increase can be realised with NIR radiation of a wavelength between 0.8 and 1.5 xcexcm. Advantages of this method are that it is contactless, eliminating the risk of mechanical damage as in case of the use of heated rollers and is highly flexible as the heating power can continuously and immediately be adapted to process needs, such as varying laminating speed, with temperature controlling devices which control the surface temperature.
In the heating step prior to the extrusion-coating the temperature of the paper is preferably raised to a value of at least 30xc2x0 C., preferably to a value between 30 and 125xc2x0 C. More in particular the upper limit of the temperature of the web is 90xc2x0 C. In case energy consumption is a consideration, the temperature may be kept at a value of less than 80xc2x0 C. without negative effects.
The extrusion-coating can be done with one layer of polymer or with a combination of polymer layers by co-extrusion. The paper web can be coated on one side or on both sides, the latter embodiment having preference. The polymers to be used for the extrusion-coating are the conventional polymers used in paper coating, more in particular in the preparation of photographic base paper. Examples are polyolefins, polyacrylates or polyesters. More in particular, the coating is done using low-density polyethylene, high density polyethylene and blends thereof. The amount of polymer coating depends on the required properties of the laminated base paper, such as stiffness, gloss, number of pits and the like. More in particular the amount is preferably not more than 50 g/m2, more preferably between 25 and 50 g/m2.
The heating step can be incorporated in an extrusion-coating Line for paper. It is to be noted that in case of the use of more than one layer of polymer resin it is possible to use co-extrusion. The term xe2x80x98extrusionxe2x80x99 is therefor intended to include also co-extrusion.
In the extrusion line also other treatments of the paper web can be used such as corona treatment, ozone treatment, flame-treatment and plasma treatment, which treatments all aim at improving the adhesion of the polymer melt to the base paper web.
The invention is now elucidated on the basis of the attached FIGURE. In this FIGURE an extrusion-coating line has been shown. The base-paper web is fed, via a number of rollers, past a corona treatment, via the nip-roll to the nip-zone. The polymer melt and the base paper web come together in the nip. The polymer melt is then cooled on the chill-roll and is released from the chill-roll at the release-roll, from where it is transported further.
Between the corona treatment and the nip-roll, NIR radiation equipment is installed. In order to get a very good efficiency, the infra-red radiation is provided from one side and a radiation mirror is provided on the other side. It is also possible to install the NIR-radiation equipment before the corona treatment.
It is to be noted that the present figure shows lamination on only one side of the paper web. In case lamination on both sides has to be provided, the partly laminated paper web can be laminated on the other side in a comparable co-extrusion line. It is also possible to include a second extruder and, if necessary, suitable radiation equipment in this line.
The invention is now elucidated on the basis of the Example.